Supercritical water has many advantages when used to upgrade heavy oil. The extent of upgrading in supercritical water processes is limited by the amount of hydrogen and the instability of catalysts in supercritical water.
The high temperature in the supercritical water reactor induces thermal cracking of chemical bonds such as carbon-sulfur bonds and carbon-carbon bonds. Broken bonds are filled with other atoms or by forming unsaturated bonds. Preferably the broken bonds are filled with hydrogen to avoid intermolecular condensation and generation of olefins and aromatics. While olefins are valuable chemicals, low stability of unsaturated bonds can degrade products by forming gums. Although hydrogen from the water molecule can participate in cracking reactions, the extent of hydrogen donation from water is limited in supercritical water condition due to high hydrogen-oxygen bond energy. Products from a supercritical water process can have higher aromaticity and olefinicity than the feed petroleum, which has a negative effect on the economic value of the products.
The harsh conditions in a supercritical water process results in unstable catalysts. Disintegration of heterogeneous catalysts is frequently observed in supercritical water. Homogeneous catalysts, such as organometallic compounds, can be transformed to inactive form under supercritical water condition. However, hydrogen abstraction from water can be benefited from in the presence of catalyst in supercritical water.